Histamine

Question 1

2. Describe histamine storage and release

Answer 1

Stored in: pre-synaptic neurons → release in awakened state → alertness

○mast cells → release in response to IgE-allergen complexes → vasodilator allowing for influx of other inflammatory mediators

○basophils → release in response to inflammation → chemotactic factor to attract basophils to site of inflammation

○enterochromaffin-like gastric cells → release in response to gastrin OR inhibited by somatostatin → stimulate HCl release from parietal cells, release pepsinogen from chief cells

Question 2

Where are H1 receptors located?

Answer 2

presynaptic neurons and mast cells

Question 3

Where are H2 receptors located?

Answer 3

presynaptic neurons and enterochromaffin-like gastric cells

Question 4

Where are H3 receptors located?

Answer 4

presynaptic neurons

Question 5

Where are H4 receptors located?

Answer 5

basophils

Question 6

List the names of common physiological antagonists used to block the adverse effects of histamine.

Answer 6

Alpha-2 and Beta-2 agonists:
Epinephrine, Ephedrine, Isoproteronol

Those that inhibit histamine release:
cromolyn and nedocromil
aminophylline and theophylline (methyxanthines like caffeine)

Question 7

What are the major side effects of first generation H1 anti-histamines?

Answer 7

sedation

Question 8

List the names of second generation H1 anti-histamines and therapeutic uses

Answer 8

Fexofenadine
Cetirizine
Loratidine

Used for allergies

Question 9

Describe the role of histamine in gastric acid secretion

Answer 9

● Gastric hyperacidity
○ GI ulcers
○ Duodenal or pancreatic gastrinoma
○ NSAID therapy
○ Hypergastrinemia secondary to renal failure-mediated uremia/hypercalcemia
○ early morning vomiting/empty stomach in dogs only fed once a day in the AM
○ H2 receptors are involved in gastric hyperacidity
○ Neuronal H2 receptors are pro-emesis by activating the CRTZ and thus H2 blocker are anti-emetics
○ Parietal cells contain H2 receptors and thus blocking these receptors prevents HCl release → minimize acidity and vomiting response.

Question 10

List the names of H2 receptor blockers and their therapeutic uses.

Answer 10

Ranitidine
Cimetidine
Famotidine
Nizatidine

Used with other drugs like sucralfate if an ulcer is present